A colt needs breaking as a youngster (10)
I believe the answer is:
adolescent
'a youngster' is the definition.
(thesaurus)
'a colt needs breaking' is the wordplay.
'breaking' is an anagram indicator.
'colt'+'needs'='coltneeds'
'coltneeds' with letters rearranged gives 'dolescent'.
'a'+'dolescent'='ADOLESCENT'
'as' is the link.
(Other definitions for adolescent that I've seen before include "Teenage" , "sixth-former, say" , "Immature" , "Youth" , "Youngster" .)